1. Field of the Invention
The present invention relates to a torque detecting apparatus for detecting rotational torque applied to a rotating shaft, and also relates to a method for assembling the same.
2. Description of the Related Art
An electric power steering apparatus have widely been used for assisting a steering operation by applying driving power of an electric motor to a steering mechanism upon responding to a turning operation of a steering member such as a steering wheel. In general, such a conventional electric power steering apparatus is provided with a torque detecting apparatus for detecting a steering torque applied to a steering member thus to control driving of the electric motor for assisting the steering. Such torque detecting apparatus is configured so that a steering shaft (rotating shaft), for connecting the steering member and the steering mechanism, is divided into two, first and second, shafts, which are coaxially connected to each other by a small diameter torsion bar, and the steering torque (rotational torque) applied to the steering shaft by the operation of the steering member can be detected as a relative angular displacement between the first shaft and the second shaft derived from a twist of the torsion bar.
As torque detecting means for detecting the relative angular displacement between the first shaft and the second shaft has been conventionally proposed in various configurations. One example of such prior art is known which is provided with a cylindrical magnet (first rotating member) fixedly fitted to an outer surface of the first shaft and a yoke ring (second rotating member) surrounding an outer circumference of the cylindrical magnet and integrally rotates with the second shaft, and configured so as to detect the relative angular displacement between the first shaft and the second shaft from a change in the magnetic circuit between the cylindrical magnet and the yoke ring (for example, referring to Japanese Patent Application Laid-open No. 2003-149062).
In the conventional torque detecting apparatus, the cylindrical magnet is configured by arranging N poles and S poles alternately in the circumferential direction thereof, and fixedly fitted to the outer surface of the first shaft. The yoke ring is configured by a pair of thin rings made of soft magnetic, sized substantially identical to each other, and provided with magnetic pole teeth which are identical in the number to the N and S poles and arranged circumferentially at equal intervals on an end edge of one side of a circular ring body of the yoke ring. The two rings are formed integral by a mold body made of a resin formed in a cylindrical shape. In the integrally formed state, the two rings are positioned so that the magnetic pole teeth of both rings are alternately arranged in the circumferential direction and each magnetic pole tooth arranged on one ring is positioned at the center between each adjoining magnetic pole teeth arranged on the other ring. The mold body is fixedly fitted onto the connecting side end portion of the second shaft by a metallic collar formed integrally with one side end portion thereof.
In a neutral state where any relative angular displacement between the first shaft and the second shaft is not present, the cylindrical magnet and the yoke ring are positioned in the circumferential direction and assembled together so that each of the magnetic pole teeth corresponds to the boundary between the N pole and the S pole of the cylindrical magnet. Accordingly, when a relative angular displacement between the first shaft and the second shaft is caused by the steering shaft being applied the steering torque, the positional relationship between the magnetic pole teeth of the two yoke rings and the magnetic poles of the cylindrical magnet is changed. As the positional relationship is changed, the magnetic flux generated in the two yoke rings changes. A change in the magnetic flux is then detected to determine the magnitude of the steering torque.
It is however necessary for assembling the conventional torque detecting apparatus to correctly position the cylindrical magnet integrally rotating with the first shaft and the yoke rings integrally rotating with the second shaft so that their positional alignment is ensured in not only the circumferential direction but also the axial direction.
The positioning in the axial direction is carried out by relatively moving the first shaft and the second shaft in the axial direction before the first shaft and the second shaft are connected to each other by the torsion bar. Nevertheless, since the cylindrical magnet and the yoke rings to be positioned are located in the housing which rotatably supports the first shaft and the second shaft, their positional relationship may hardly be acknowledged by view. Thus, conventionally, positioning in the axial direction was taken as below. At first, while the second shaft is positioned and supported in the housing, the first shaft is lowered and inserted until the end portion of the cylindrical magnet fixedly fitted onto the first shaft collides against the second shaft or the yoke rings formed integrally with the second shaft. Then, using the collision position as a reference position, the first shaft is moved back or lifted up in a direction of departing from the second shaft by a predetermined distance.
However, the cylindrical magnet fixedly fitted onto the first shaft includes a group of magnetic segments of a rectangular shape which are arranged in the circumferential direction and assembled together with the mold body of a resin in a cylindrical shape. As the cylindrical magnet is fixedly fitted onto the first shaft by the mold body, the mold body of a resin may be injured thus to have, for example, cracks or flaws by the first shaft colliding against the second shaft for determining the reference position during the positioning operation described above.
When the cylindrical magnet integrally formed with the mold body having any fault which is not acknowledged is positioned in both the axial direction and the circumferential direction and assembled to the apparatus, its fixing strength remains low thus permitting the accuracy of the torque detection to be declined. Such a drawback may be lessened by minimizing the collision between the two shafts for determining the reference position. It is not yet assured that the mold body is protected from any fault, while the assembling process becomes more intricate.
As explained, any other conventional torque detecting apparatus in an electric power steering apparatus is similarly arranged for detecting a relative angular displacement between the first shaft and the second shaft connected to each other by the torsion bar with the use of various detecting means. Thus, since any other conventional torque detecting apparatus is adapted for detecting a relative angular displacement between a first rotating member fixedly fitted onto the first shaft and a second rotating member integrally rotating with the second shaft, its relative positioning in the axial direction of the two rotating members will encounter the foregoing drawback.